1. Technical Field
Embodiments of the present invention relate to a sheet conveying device that includes at least a pair of sheet holding/conveying belts, an image forming apparatus incorporating the sheet conveying device, and a method of conveying a sheet in the image forming apparatus including the sheet conveying device.
2. Related Art
Known image forming apparatuses include a configuration to form a toner image on a sheet-like recording material using electrophotography and to fuse and fix toner to the recording material by causing the recording material to pass through a thermal fixing device. When an image output operation is serially performed and the recording materials are loaded one after another in a sheet discharging part after passing through the thermal fixing device, the toner on the recording material may not harden sufficiently. If the toner is still soft, the toner on the recording material can adhere to a subsequent or different recording material. This defect in printing is referred to as blocking. When blocking occurs, the quality of image formation decreases significantly.
To address this inconvenience, it has been proposed to provide electrophotographic image forming apparatuses including a cooling device therein. The cooling device provides a method of cooling a recording material directly by intaking air from outside with fans or a method of cooling a recording material by causing the recording medium to contact a cooled heat exchanger directly or via a thermal conducting member. By using this configuration, heat exchange is conducted between the heat exchanger and the recording material.
Along with the recent increase in speed of printing, the cooling device increases in size. To cool the toner on the recording material more effectively, Japanese Patent Application Publication No. JP 2012-167844-A discloses a cooling device that includes a pair of endless belts disposed vertically in an image forming apparatus. An air-cooled or liquid-cooled heat exchanger is disposed on an inner circumference of one of the belts to cool the recording material.
However, non-contact areas on the belt in a vicinity of an entrance and an exit of the cooling device do not contact the heat exchanger, and therefore have a higher temperature than contact areas where the heat exchanger contacts the belt. Accordingly, the toner on the non-contact areas is difficult to be hardened, in other words, the toner remains soft. Specifically, the non-contact area on the belt that is separated from a heat exchanger disposed at an extreme downstream side in a sheet conveying direction approaches the entrance of the cooling device disposed closest to the fixing device along with rotation of the belt. Consequently, in the event of a paper jam anywhere in paper paths in the image forming apparatus during an image forming operation (a printing operation), if the recording material remains in the vicinity of the entrance or the exit of the cooling device for a long period, the fresh toner that remains soft can adhere to the belt. Even after the paper jam is cleared, if the image forming operation is resumed in this status, toner sticking marks or streaks are formed on the image formed on the recording material, which results in production of a defect image.